In recent years, as electronic equipment is seeing the ever-increasing level of size reduction and circuit density, a shift from conventional one surfaced substrates to two-surfaced substrates and multilayer substrates has been taking place in the area of circuitry forming substrates for mounting electronic components thereon, resulting in stepped up development of a high density circuitry forming substrate that enables the mounting of as many circuits as possible thereon.
With a high density circuitry forming substrate, in place of the conventional method of drilling to bore a hole (through hole) on a substrate, it is being considered to use energy beams such as a laser beam and the like, whereby machining at a higher speed and with a higher degree of definition is made possible. (cf. An article titled “Remarkable Recent Trend in Development of Build-up Multilayer PWB” authored by K. Takagi and appeared in “Surface Mounting Technology” published in January 1997 from Nikkan Kogyo Shinbun of Japan)
However, with a high density substrate, as the line width and land size of circuits are reduced, the adhesion strength between the substrate and the circuits or the lands is naturally reduced. On the other hand, a high density substrate is usually used in such electronic devices as note PC's, portable telephones, PDA's and the like, and is liable to suffering from such mechanical stresses as caused by dropping, bending, impacting and the like occurring on the part of the electronic devices. As a result, the aforementioned reduction in adhesion strength of the high density substrate has been making a serious problem.
As the substrate material for a high density substrate, most of the time, a resin material is used and a high grade thermosetting resin is usually used from the view points of enhancement in the mechanical strength or heat resistance of the substrate. Although the properties of the substrate itself may be improved, however, the adhesion strength between the substrate and the circuits or the lands is not necessarily intensified when such mechanical stresses as an impact and the like imposed on the substrate are taken into consideration.
Conversely, when a material design and a material selection are carried out for the whole purpose of increasing the adhesion strength, the substrate properties except for the adhesion strength are often degraded. Therefore, it is actually difficult for the substrate material development to be performed so as to satisfy all the requirements involved with a high density substrate.
In addition, with a high density circuitry forming substrate, such connecting means as a conductive paste, a metal plating and the like are used in electrically connecting between layers of the substrate and, on the other hand, a spacing between through holes or non-through holes, which are generally referred to as via holes and formed on the substrate for interlayer connections, and a spacing between the via hole and the adjoining wiring are made smaller and smaller, resulting sometimes in such a serious problem of reliability for the circuitry forming substrate as caused by diffusion of a conductive paste and a plating solution for metal plating to the vicinity of via holes.